Our long term goal of this study is to elucidate the roles of annexins II, V and VI and the interactions between annexin V and types II and X collagen in terminal differentiation events of chondrocytes. Terminal differentiation of growth plate chondrocytes consists of a series of events including mineralization and programmed cell death (apoptosis). These events play a crucial role during normal bone formation. If they, however, occur during pathological conditions, such as osteoarthritis, they will lead to cartilage destruction. Thus, an understanding of the cellular mechanisms controlling terminal differentiation of chondrocytes is of great importance. Major advances during the last funding period led to the following three new hypotheses which will be tested in the current proposal: (i) annexin channel formation in the plasma membrane of growth plate chondrocytes and annexin V channel activation by types II and X :ollagen lead to calcium influx into growth plate chondrocytes and alteration of calcium homeostasis; (ii) optimal annexin channel formation and annexin V/collagen interactions require the interactions between annexin II, V and VI; (iii) annexin-mediated alteration of calcium homeostasis regulates terminal differentiation events of growth plate chondrocytes. These hypotheses will be addressed through the following specific aims: 1. We will determine the function of annexin II, V and VI channel formation and annexin V/collagen interactions in alteration of calcium homeostasis in growth plate chondrocytes and test whether annexins through binding to collagen and cytoskeleton act as mechanosensors in these cells. 2. We will determine the interactions between annexin II, V and VI as a possible major regulator of annexin channel formation and annexin V/collagen interactions. 3. We will determine the regulatory roles of annexins and annexin V/collagen interactions in terminal differentiation events including mineralization and apoptosis of growth plate chondrocytes.This study relates directly to the mechanisms which control terminal differentiation of chondrocytes and investigates a novel mechanism regulating calcium homeostasis in skeletal cells.Thus, this proposal will not only greatly advance our understanding of how terminal differentiation events are regulated, but it might also provide novel therapeutic targets to prevent terminal differentiation events during patholoaical conditions.